Recent Greenhouse Gas Concentrations

DOI: 10.3334/CDIAC/atg.032

Updated February 2014

Investigator

T.J. Blasing

Gases typically measured in parts per million (ppm), parts per billion (ppb) or parts per trillion (ppt) are presented separately to facilitate comparison of numbers. Global Warming Potentials (GWPs) and atmospheric lifetimes are from the Intergovermental Panel on Climate Change (IPCC, 2007, Table 2.14), except for the atmospheric lifetime of carbon dioxide (CO2) which is explained in footnote 4. Additional material on greenhouse gases can be found in CDIAC's Reference Tools. To find out how CFCs, HFCs, HCFCs, and halons are named, see Name that compound: The numbers game for CFCs, HFCs, HCFCs, and Halons. Concentrations given apply to the lower 75-80 percent of the atmosphere, known as the troposphere.

Sources of the current concentrations are given in the footnotes. The concentrations given are mostly derived from data available via the CDIAC Web pages; many corresponding links are given in the footnotes below. These data are contributed to CDIAC by various investigators, and represent considerable effort on their part. We ask as a basic professional courtesy that you acknowledge the primary sources when you refer to data from any of these sites. Guidelines for proper acknowledgment are found at each link, except for the ALE/GAGE/AGAGE database where acknowledgment guidelines are given in the "readme" files; links to those "readme" files are given in footnote 9, below. Concentrations of ozone and water vapor are spatially and temporally variable due to their short atmospheric lifetimes. A vertically and horizontally averaged water vapor concentration is about 5,000 ppm. Globally averaged water vapor concentration is difficult to measure precisely because it varies from one place to another and from one season to the next. This precludes a precise determination of changes in water vapor since pre-industrial time. However, a warmer atmosphere will likely contain more water vapor than at present. For a more detailed statement on water vapor from the National Oceanic and Atmospheric Administration, see http://lwf.ncdc.noaa.gov/oa/climate/gases.html

GAS

Pre-1750 tropospheric concentration1

Recent tropospheric concentration2

GWP3(100-yr time horizon)

Atmospheric lifetime4(years)

Increased radiative forcing 5 (W/m2)

Concentrations in parts per million (ppm)

Carbon dioxide (CO2)

2806

395.47

1

~ 100-3004

1.88

Concentrations in parts per billion (ppb)

Methane (CH4)

7228

18939/17629

28

124

0.49

Nitrous oxide (N2O)

27010

3269/3249

265

1214

0.17

Tropospheric ozone (O3)

2371

3372

n.a.3

hours-days

0.40

Concentrations in parts per trillion (ppt)

CFC-11 (trichlorofluoromethane) (CCl3F)

zero

2369/2349

4,660

45

0.061

CFC-12 (CCl2F2)

zero

5279/5279

10,200

100

0.169

CF-113(CCl2CClF2)

zero

749/749

5,820

85

0.022

HCFC-22(CHClF2)

zero

2319/2109

1,760

11.9

0.046

HCFC-141b(CH3CCl2F)

zero

249/219

782

9.2

0.0036

HCFC-142b(CH3CClF2)

zero

239/219

1,980

17.2

0.0042

Halon 1211 (CBrCIF2)

zero

4.19/4.09

1,750

16

0.0012

Halon 1301 (CBrCIF3)

zero

3.39/3.39

6,290

65

0.0010

HFC-134a(CH2FCF3)

zero

759/649

1,300

13.4

0.0108

Carbon tetrachloride (CCl4)

zero

859/839

1,730

26

0.0143

Sulfur hexafluoride (SF6)

zero

7.799,11/7.399,11

23,500

3200

0.0043

Other Halocarbons

zero

Varies by substance

collectively <0.02

Footnotes

Preindustrial (1750) concentrations of CO2, CH4, N2O are taken from Chapter 8.3.2 of IPCC (2013). Global-scale trace-gas concentrations from prior to 1750 are assumed to be practically uninfluenced by human activities such as increasingly specialized agriculture, land clearing, and combustion of fossil fuels. However, effects of agriculture are possibly responsible for the increase in methane concentration around 1800 and perhaps some of the much smaller increases that occurred earlier. Preindustrial concentrations of industrially manufactured compounds are given as zero. The short atmospheric lifetime of ozone (hours-days) together with the spatial variability of its sources precludes a globally or vertically homogeneous distribution, so that a fractional unit such as parts per billion would not apply over a range of altitudes or geographical locations. Therefore a different unit is used to integrate the varying concentrations of ozone. The total mass of ozone in the troposphere is estimated in units of teragrams (Tg). A Tg is 1012 grams, or a million metric tons. Preindustrial and recent O3 amounts are taken from Chapter 8.2.3.1 of IPCC (2013).

Because atmospheric concentrations of most gases tend to vary systematically over the course of a year, figures given represent averages over a specific 12-month period for all gases except ozone (O3), for which a current tropospheric total amount has been more broadly estimated (IPCC, 2013, page 670). The CO2 concentration given is the average for year 2013, taken from the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, website maintained by Dr. Pieter Tans. For other chemical species, the values given are averages for 2012; data are found on the CDIAC AGAGE page or the AGAGE home page.

The Global Warming Potential (GWP) provides a simple measure of the radiative effects of emissions of various greenhouse gases, integrated over a specified time horizon, relative to an equal mass of CO2 emissions. The GWP with respect to CO2 is calculated using the formula:

where ai is the instantaneous radiative forcing due to the release of a unit mass of trace gas, i, into the atmosphere, at time TR, Ci is the amount of that unit mass remaining in the atmosphere at time, t, after its release and TH is TR plus the time horizon over which the calculation is performed (100 years in this table). The formula is adapted from page 210 of IPCC (2007). The GWPs given are from Table 8.A.1 of IPCC (2013). The short lifetime of ozone (hours-days) precludes a meaningful calculation of global warming potential on the time horizons (20, 100, and 500 years) listed in IPCC documents.

The atmospheric lifetime is used to characterize the decay of an instantaneous pulse input to the atmosphere, and can be likened to the time it takes that pulse input to decay to 0.368 (l/e) of its original value. The analogy would be strictly correct if every gas decayed according to a simple exponential curve, which is seldom the case. For example, CH4 is removed from the atmosphere by a single process, oxidation by the hydroxyl radical (OH), but the effect of an increase in atmospheric concentration of CH4 is to reduce the OH concentration, which, in turn, reduces destruction of additional methane, effectively lengthening its atmospheric lifetime. An opposite kind of feedback may shorten the atmospheric lifetime of N2O (IPCC 2007, Section 2.10.3). For CO2 the specification of an atmospheric lifetime is complicated by temporary removal processes which store carbon in the biosphere before it is returned to the atmosphere as CO2 via respiration or, as a combustion product, in fires. This necessitates complex modeling of the decay curve. Because the modelled decay curve depends on the model used and the assumptions incorporated therein, it is difficult to specify an exact atmospheric lifetime for CO2. Most estimates fall in the 100-300-year range. The above-described processes are all accounted for in the derivation of the atmospheric lifetimes given in the above table, taken from Table 8.A.1 in IPCC (2013).

Changes in radiative forcing since 1750 represent changes in the rate per square meter, at which energy is supplied to the atmosphere below the stratosphere. Note from Figure TS.6 (top) in the Technical Summary of IPCC (2013) that aerosols frequently have the effect of decreasing this radiative forcing. Energy is measured in Joules; the rate at which it is made available is in Joules/second, or Watts; hence, radiative forcing is measured in Watts per square meter (W/m2). The value for increased radiative forcing of CO2 is based on the 2013 concentration and the 1750 concentration given in the above table. Values for CH4 and N2O are based on AGAGE global average concentrations for 2012 and the 1750 values given. Radiative forcing for tropospheric ozone is taken from the 5th column of Table 8.6 of IPCC (2013). The "current" value refers to a global average. Note, in the row immediately below the number for tropospheric forcing, the stratospheric forcing is given as negative 0.05 W/m2. Note also the uncertainty ranges given in the tables. For CH4, N2O, and gases expressed in concentrations of parts per trillion, radiative forcings apply to average global concentrations given by AGAGE for 2012, and are based on the radiative efficiencies given in Table 8.A.1 of IPCC 2013. For the gases expressed in parts per trillion, it is assumed that radiative forcing prior to 1750 was zero because their only source is manufacture after that time. Calculations for these gases assume that the radiative efficiencies have not changed with time, for these small concentrations (cf. Mitchell 1989). The upper bound for the collective value of radiative forcing increases given for the "other halogenated species" was approximated by subtracting the sum for those listed from the total for all halogens given in Table 8.2 of IPCC (2013), and rounding the result upward. Radiative forcing estimates of one investigator may differ slightly from those another due to differences in assumed preindustrial values, radiative efficiencies, or values used as recent atmospheric concentrations. For comparison, see the radiative forcings given by the National Oceanic and Atmospheric Administration (NOAA) at NOAAs Annual Greenhouse Gas Index site, which also gives the equations used in the calculations of radiative forcing.

Blasing (1985) gave the range of best estimates of the CO2 mole fraction around year 1800 as between 275 and 285 parts per million. This was drawn from an extensive study of previous work by Gammon et al. (1985), which gave that range from within a broader possible range of 260-285 ppm. IPCC (2013, Technical Summary, page 50) gives a range of 273-283 ppm for year 1750; Chapter 8 of IPCC (2013) indicates a narrower range of 276-280. The Law Dome Ice core record available on the CDIAC web site, indicates a value of 277 for year 1750; IPCC (2013) gives 278 ppm. These values are generally consistent with those from Neftel et al. From all this we conclude that estimates of preindustrial concentrations have been robust as new information has been obtained over the last 30 years or more. The slight differences from one persons estimate to the next lead to slight differences in estimated increases in radiative forcing since "preindustrial times" which are now taken as the radiative forcings in year 1750. Evidence of pre-industrial CO2 concentrations comes from several sources, including well-dated carbon-isotope signatures, in annual tree rings (Stuiver et al. 1984). Estimates of "pre-industrial" CO2 can also be obtained by first calculating the ratio of the recent atmospheric CO2 increases to recent fossil-fuel use, and using past records of fossil-fuel use to extrapolate past atmospheric CO2 concentrations on an annual basis. Estimates of "pre-industrial" CO2 concentrations obtained in this way are higher than those obtained by more direct measurements; this is believed to be because the effects of widespread land clearing are not accounted for. Ice-core data provide records of earlier concentrations. For over 400,000 years of ice-core record from Vostok, see J. M. Barnola et al. For ice-core records extending 800,000 years back in time, see CDIACs Gateway Page to CO2 data.

Pre-industrial concentrations of CH4 are evident in the 2000-year records from Law Dome, Antarctica and longer ice-core records found on CDIAC's collection of data access links to atmospheric trace gases. A spline function fit to those data gives 697 ppm for year 1750, but this may be lower than the global average if agricultural sources in the Northern Hemisphere were already contributing nontrivially. For graphs of two-thousand-year records of CH4, CO2 and N2O concentrations are found here.

The first value in a cell represents Mace Head, Ireland, a mid-latitude Northern-Hemisphere site, and the second value represents Cape Grim, Tasmania, a mid-latitude Southern-Hemisphere site. "Current" values given for these gases are annual arithmetic averages based on monthly background concentrations for year 2012. Source: Advanced Global Atmospheric Gases Experiment (AGAGE) data posted on the AGAGE/CDIAC web site. These data are compiled from data on finer time scales in the ALE/GAGE/AGAGE database (Prinn et al., 2000), and represent the work of several investigators at various institutions; guidelines on citing the various parts of the AGAGE database are found here or within the ALE/GAGE/AGAGE database, which can also be accessed via anonymous ftp.

The value given for 1750, obtained from a spline fit to measured values in the ice core record from Law Dome, Antarctica, is 271 ppb.